KH:

We do not publicize how often the no-fly system stops people you would not want on your flight. Several times a week would low-ball it.

Almost 20,000 False Positives:

The Justice Department's proposed budget for 2008 reveals for the first time how often names match against the database, reporting that there were 19,967 "positive matches" in 2006.

19,967 / 52 = 383.9
What that really translates to approximately 350 people per week inconvenienced. If they were arrested, deported, or their plot foiled we would of heard about it in the news.

But remeber what KH said about why they use the no-fly list:

Because it works.

Schneier on Security: Conversation with Kip Hawley, TSA Administrator (Part 3)